1. Technical Field of the Invention
The present invention relates to a wet etching method and a wet etching apparatus. More particularly, it relates to a technique to control characteristics of an etching liquid used in wet etching of objects to be processed, such as semiconductor wafers.
2. Description of the Related Art
FIGS. 9A and 9B are views for explanation of an etching process for the semiconductor wafers. FIG. 9A shows a wafer W before etching. On a surface of a silicon substrate 1 as base metal, a silicon oxide layer 2 (SiO2) and a silicon nitride layer 3 (Si4N3) are laminated as undercoating layers, in sequence. on the surface of the silicon nitride layer 3, a resist film 4 is applied in the form of a designated patter. (see FIG. 9A) The wafer W is immersed into a phosphate solution (H3PO4 aq.) of 160 to 180xc2x0 C. in a not-shown process bath and etched as shown in FIG. 9B. In the etching process, it is important to control a layer thickness of the silicon oxide layer 2 underlying the silicon nitride layer 3.
Japanese Examined Patent Publication (kokoku) No. 3-20895 discloses an example of the wet etching apparatus. In the apparatus, the etching liquid is stored in the process bath and heated up to 160 to 180xc2x0 C. in temperature. Connected to the process bath is a circulating pipe which includes a circulation pump, a filter, a temperature controller, etc. The etching liquid is controlled at a predetermined temperature by its circulation through the circulating pipe.
When repeating the etching process, it causes the concentration of silicon (Si) in the phosphate solution to be increased to produce oxides and particles. Consequently, the filters interposed in the circulating pipe are choked by the above substances. Thus, in the etching process, the etching liquid in the process bath has been exchanged in its entirety at regular intervals.
However, it should be noted that the etching rate against the oxide layer is extremely high immediately after the etching liquid, i.e. the phosphate solution (H3PO4 aq.) has been exchanged for new one. Thus, as shown in FIG. 10, the silicon oxide layer 2 underlying the silicon nitride layer 3 is excessively etched (over-etching) to produce a recess 5, whereby it is difficult to control the thickness of oxide layer.
In order to solve the above problem, it has been studied to shorten a processing period after exchanging the etching liquid or control the etching rate against the oxide layer, for example, by immersing a dummy wafer made of silicon into the etching liquid in view of elevating the concentration of silicon intentionally. In the former method of shortening the processing period, however, it is impossible to fully control the layer thickness of oxide layer because of the difficulty in managing the processing period. While, in the latter method of raising the concentration of silicon intentionally, the wafers have been wasted from the exchange of the etching liquid till the etching capable of the desired layer thickness, so that the yield of products falls.
Again, as the etching liquid is to be used at high temperature of 160 to 180xc2x0 C., there is also a problem raised of wasting much time in order to elevate the temperature of the etching liquid after it has been exchanged. Additionally, the problem is accompanied by high consumption of the etching liquid.
Accordingly, it is therefore an object of the present invention to provide a method of and an apparatus for etching, which is capable of simplifying the etching control and improving the yield of products and by which it is possible to make use of the etching liquid effectively and shorten a period required for exchanging the same.
In order to attain the above object, according to the first aspect of the present invention, there is provided an etching method including the steps of: etching an object to be processed by immersing the object in an etching liquid stored in a process bath; draining a predetermined amount of the etching liquid used in the etching step from the process bath while leaving the remaining amount of the etching liquid; and supplying a new etching liquid into the process bath to add the new etching liquid to the remaining etching liquid.
According to the present invention, since the predetermined amount of etching liquid used for the etching step is discharged while leaving a part of the liquid in the process bath and the new etching liquid is supplied into the process bath, it is possible to shorten the time required for discharging and charging the etching liquid. Thus, with the reduction in the exchange period of the etching liquid, it is possible to use the etching liquid effectively.
Additionally, since the object to be processed is not subjected to over-etching just after exchanging the etching liquid, it is possible to control a layer thickness of the object with ease, whereby the yield of products can be improved.
Moreover, since the new etching liquid is added to the etching liquid of high temperature which is remaining after discharging the etching liquid of the predetermined amount, it is possible to shorten a period required for elevating the temperature of the etching up to a predetermined temperature capable of effective etching, whereby the reduction in energy consumption can be accomplished.
According to the second aspect of the present invention, there is also provided an etching apparatus including: a process bath for accommodating an object to be processed therein and storing an etching liquid for etching the object; a drain system for draining the etching liquid in the process bath, the drain system being capable of adopting a draining state in which the drain system is draining the etching liquid and a non-draining state in which the drain system stops draining the etching liquid; a supply system for supplying a new etching liquid into the process bath, the supply system being capable of adopting a supplying state in which the supply system is supplying the etching liquid and a non-supplying state in which the drain system stops supplying the new etching liquid; and control means for controlling an operation of the drain system to drain a predetermined amount of the etching liquid in the process bath while leaving the remaining amount of the etching liquid and an operation of the supply system to supply the new etching liquid into the process bath, when the concentration of a specified substance contained in the etching liquid in the process bath reaches a predetermined value.
The above and other features and advantages of the present invention will become apparent and the invention itself will best be understood with reference to the attached drawings showing a preferred embodiment of the invention.